UvA-DARE (Digital Academic Repository)
Mass loss and evolution of asymptotic giant branch stars in the Magellanic Clouds
van Loon, J.T.
Link to publication
Citation for published version (APA):
van Loon, J. T. (1999). Mass loss and evolution of asymptotic giant branch stars in the Magellanic Clouds
General rights
It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s),
other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons).
Disclaimer/Complaints regulations
If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating
your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask
the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam,
The Netherlands. You will be contacted as soon as possible.
UvA-DARE is a service provided by the library of the University of Amsterdam (http://dare.uva.nl)
Download date: 15 Jun 2017
Chapterr 6
Discoveryy of H2O maser emission from
thee red supergiant IRAS04553-6825 in
thee Large Magellanic Cloud
JaccoJacco Th. van Loon, Peter te Lintel Hekkert, Valentin Bujarrabal, Albert A. Zijlstra, Lars-Ake
Nyman,Nyman, 1998, A&A 337, 141
Wee report the detection of 22 GHz H 2 0 maser emission from the red supergiant IRAS04553-6825
inn the Large Magellanic Cloud. It is the first known source of circumstellar H 2 0 maser emissionn outside the Milky Way. The measured flux density is comparable to that expected from
scalingg the galactic red supergiant NML Cyg. The peak velocity agrees with the SiO maser
peakk velocity.
AA near-infrared spectrum indicates that IRAS04553-6825 has a typical LMC metallicity.
Wee argue that, possibly as a result of the low metallicity, the H 2 0 emission probably occurs
nearr or within the dust formation radius, rather than further out as appears to be the case in
NMLL Cyg and galactic OH/IR stars.
6.11
Introduction
Maserr emission from evolved stars is associated with dusty, oxygen rich circumstellar envelopes
(CSEs),, mainly around red supergiants (RSGs) and stars at the tip of the Asymptotic Giant
Branchh (AGB) (Elitzur 1992; Habing 1996). The strongest maser lines originate from OH (at
16122 MHz), H 2 0 (at 22 GHz) and SiO (at 43 and 86 GHz), located progressively deeper into
thee CSE. Hence these lines are used to trace the mass-loss history and velocity structure of
thee CSE, through their photon fluxes and line profiles (Chapman & Cohen 1986; Lewis 1989,
1990).. Maser lines are also excellent tracers of the stellar space velocity and have been used to
mapp the kinematics and mass distribution in our Galaxy (e.g. Lindqvist et al. 1992; Sevenster
ett al. 1995).
Circumstellarr OH and SiO maser emission has recently been detected in the Large Magellanicc Cloud (LMC) (Wood et al. 1986, 1992; van Loon et al. 1998a, 1996: Chapters 3 & 5).
Woodd et al. (1992) argue that the outflow velocities in CSEs of OH/IR stars in the LMC are
aa factor two lower than in corresponding Milky Way stars, a difference they attribute to less
efficientt driving of the outflow caused by the lower metallicity in the LMC. However, Zijlstra
75 5
76 6
ChapterChapte
ett al. (1996) find the difference to be only 20 to 30%, from their analysis of the same data.
Moreover,, the SiO maser emission from the LMC RSG IRAS04553-6825 (Chapter 5) showed
thatt in this case, the outflow velocity as derived from the OH maser profile was underestimated
byy more than a factor two. The relation between outflow velocity and metallicity is therefore
stilll uncertain.
Inn the LMC, SiO maser emission is generally too faint to be detected, with IRAS04553-6825
thee only successful detection to date. Stellar H 2 0 maser emission has not been found outside
thee Galaxy before.
Heree we present the discovery of the first extra-galactic source of circumstellar H 2 0 maser
emissionn — viz. the RSG IRAS04553-6825 in the LMC. We also present spectroscopic results
thatt confirm the LMC metallicity of its stellar photosphere. We then discuss the implications
forr the structure of the CSE by comparing with the galactic red supergiant NML Cyg.
6.22
Observations and Results
6.2.11
H 2 0 radio observations
Thee observations were performed on August 19, 20 of 1997, using the 64 m radio telescope
att Parkes, Australia. We observed the 6i 6 -» 5 23 rotational transition of ortho-H 2 0 at a rest
frequencyy of 22.235 GHz, using the 1.3 cm receiver plus autocorrelator backend. The 64 MHz
bandd width and 1024 channels centred at ~22.21 GHz yield a velocity coverage of ~860 km s _1
att 0.84 km s _ 1 channel -1 . Using the Dual Circular feed we simultaneously obtained spectra
inn left and right circular polarization. The beam FWHM is 1.4', the system temperature is
typicallyy 110 K, and the conversion factor from antenna temperature to flux density is 6.3 Jy
K" 1 .. The nearby sky was measured every two minutes, resulting in very flat baselines that
requiredd only a very shallow second-order polynomial to be subtracted.
Wee observed the LMC RSG IRAS04553-6825 for six hours on-source integration. No differencee was found between the spectra obtained at either polarization, which we then averaged.
Thee final 22-GHz spectrum of IRAS04553-6825 is presented in Fig. 5.1, with flux densities in
mJyy and heliocentric velocities in km s'1. The measured rms (1 a) is only 5.5 mjy.
Wee detected a single peak of H 2 0 maser emission with a peak flux density of 68 mJy
(withh 20% calibration accuracy), corresponding to a 12 a detection. The peak is centred at a
heliocentricc velocity of 277 km s _ 1 and has a FWHM of 1.7 km s - 1 . The integrated flux of the
emissionn is 0.17 Jy km s~ l .
6.2.22
Call triplet echelle spectroscopy
Wee used the 3.5 m New Technology Telescope (NTT) at the European Southern Observatory
(ESO)) at La Silla, Chile, on January 7,1996, with the ESO Multi-Mode Instrument (EMMI) to
obtainn an echelle spectrum of IRAS04553-6825. Grating #14 and grism # 4 as cross disperser
weree used, yielding a spectral coverage of 6000 to 9000 A. The slit width and length were 2"
andd 4", respectively. The integration time was one hour.
Thee data were reduced in the normal way using the Munich Interactive Data Analysis
Softwaree (MIDAS) package. The wavelength calibration was done by taking a ThAr lamp
spectrumm in conditions identical to the spectrum of IRAS04553-6825. The measured spectral
resolvingg power is R ~ 4 x 104.
77 7
DiscoveryDiscovery of H20 maser emission from IRAS04553—6825
ii
r-Tinn
pF
OH H
>»» 60
BB
ww
cc
0) )
XX
40 0
20 0
^1fMJ
1 J
00
20 0
2400
320 0
260
280
300
heliocentricc velocity (km/s)
Figuree 6.1: Discovery spectrum of the 22 GHz H 2 0 maser emission from LMC red supergiant
IRAS04553-6825.. The velocities are heliocentric. Also indicated are the peak velocities of the
866 GHz SiO and 1612 MHz OH masers (arrows) and the total velocity extent of the 1612 MHz
OHH maser (bar).
w a v e l e n g t hh (A)
8 5 0 00
l
I I M
8505
8510
ll
Caa II
8498.016 6
8545
8550
T — i — I — i — i — i — i — | — ii
8555 8665
8670
8675
r
Caa II
8542.087 7
-0.55
1000 2 0 0 300 4 0 0
1000 200 3 0 0 400
100 2 0 0 3 0 0 4 0 0
h e l i o c e n t r i cc v e l o c i t y ( k m / s )
Figuree 6.2: Echelle spectra of the Ca n triplet lines in IRAS04553-6825.
Thee equivalent width of the Ca II triplet lines at 8498 8542, and 8662 A measures the surface
gravityy in giants and supergiants, if the metallicity is known. Reversely, if an estimate of the
surfacee gravity is available from knowledge of spectral type and luminosity, the Ca n lines can
bee used to infer the metallicity.
Thee echelle spectrum of IRAS04553-6825 around the Ca II triplet is presented in Fig. 6.2,
78 8
ChapterChapte
onn an arbitrary flux scale. The equivalent widths of the two strongest components are measured
ass W8542 = 1.6 0.1 A and W8662 = 1.7 0.1 A. Their sum is W8542+8662 = 3.3 0.2 A.
Thee Ca II absorption is maximum at a heliocentric velocity of ~300 km s~ l , corresponding
too a redshift with respect to the stellar velocity of a little less than the outflow velocity in
thee outer parts of the CSE. This is explained by scattering of the photospheric spectrum by
ann extended, expanding dust shell. The measured equivalent widths of the Ca II lines are not
affectedd by the scattering (see Romanik & Leung 1981, and references therein).
Forr IRAS04553-6825 we adopt a bolometric luminosity of L ~ 4 x 105 L 0 , and an effective
temperaturee between 2500 and 3000 K (spectral type: M7 supergiant). The surface gravity
mustt then be about log g ~ 0 cm s~2. If we then compare the measured equivalent width of
thee Ca 11 triplet with the results from Garcia-Vargas et al. (1998), we find that consistency is
onlyy achieved if the metallicity of IRAS04553-6825 is a factor two to three lower than solar.
Thiss result is not very sensitive to the values of the adopted stellar parameters, and hence we
concludee that IRAS04553-6825 has a typical LMC metallicity (Z = 0.008).
6.33
Discussion
6.3.11
Circumstellar H 2 0 masers in the LMC
Thee detection of IRAS04553-6825 shows that H 2 0 masers in the LMC with photon fluxes of
10444 s _ 1 are detected at a 5 a level within 6 hours of on-source integration, with the current
receiverr at Parkes. Typical observed H 2 0 maser photon fluxes from galactic sources are between
10433 s" 1 (Lindqvist et al. 1990) and 1044"45 s"1 (Nyman et al. 1986) for OH/IR stars, 1041~44 s" 1
forr irregular and semiregular red variables with mass-loss rates ~ 10~7 M 0 yr _1 (Szymczak h
Engelss 1997), 10 43-44 s _ 1 for Mira variables (Benson & Little-Marenin 1996; Yates et al. 1995)
andd 10 44-45 s _ 1 for supergiants (Yates et al. 1995). This implies that many of the mass-losing
RSGss and AGB stars (i.e. IRAS point sources) in the LMC are expected to have detectable
H2OO maser emission.
6.3.22
Introduction to IRAS04553-6825
Withh a bolometric luminosity Mho\ = -9.3 mag and a spectral type M7, IRAS04553-6825 was
recognisedd to be the most luminous (very) red supergiant in the LMC by Elias et al. (1986).
Thee progenitor mass was estimated MZAMS ~ 50 MQ by Zijlstra et al. (1996). The current
mass-losss rate of M ~few 10"4 MQ yr _1 was estimated from the self-absorbed 10 fxm silicate
dustt feature by Roche et al. (1993), who argue that the lower than expected extinction in the
opticall indicates a disky CSE.
IRAS04553-68255 exhibits strong OH 1612 MHz and weaker 1665 MHz mainline emission
(Woodd et al. 1986, 1992). Comparison of the SiO maser peak velocity and the OH maser
emissionn profile revealed that OH is only observed from the blue-shifted part of the shell. The
outfloww velocity is 27 km s" 1 , similar to Milky Way RSGs. (Wood et al. derived 10 km s _1 as is
suggestedd by the OH maser emission profile alone.) It is possible that the red-shifted emission
iss much weaker than the blue-shifted and therefore below the detection limit. Stronger blueshiftedd emission is expected if the maser amplifies emission from within the shell. Alternatively,
ann asymmetric and structured OH profile can also arise from bipolar outflow (Chapman 1988;
tee Lintel Hekkert et al. 1988).
DiscoveryDiscovery of H20 maser emission from IRAS04553-6825
79 9
Tablee 6.1: Comparison between the properties of the red supergiants NML Cyg in the Milky
Wayy and IRAS04553-6825 in the LMC.
IRAS04553-6825 5
NMLL Cyg
6
44 x 105 L 0
Bolo.. Luminosity 55 x 10 L 0
500 kpc
Distance e
22 kpc
~500 M 0
Progenitorr Mass ~5OM 0 0
M7 7
Spectrall Type
M6 6
9300 days
Pulsationn Period 9400 days
1.8xl0" 44 M 0 yr" 1
~5xl0~ 44 M 0 yr" 1
Mass-losss Rate
emission+absorption n emission+absorption n
100 /im Silicate
1.99 mag
(K(K — L) colour 2.44 mag
1.22 mag
{H{H - K) colour 2.00 mag
277 km s"1
Outfloww Velocity 288 km s" 1
diskk 4- bipolar
diskk 4- bipolar
CSEE Geometry
bluee asymm./duplicity bluee asymm./duplicity
OHH Profile
1.00 kJy
OHH Peak (1 kpc) 1.11 kJy
3.77 x 1045 s- 1
OHH Photon Flux 44 x 1045 s- 1
peakk + broad comp. peakk 4- broad comp.
SiOO Profile
2700 Jy
SiOO Peak (1 kpc) 1444 Jy
2.66 x 1045 s"1
SiOO Photon Flux 0.66 x 1045 s" 1
Fsio/Ffym Fsio/Ffym
F int /F max (SiO) )
H 2 00 Profile
H 2 00 Peak (1 kpc)
H 2 00 Photon Flux
6.3.33
ll
90 0
ll
43 3
1
7.33 km sblue-shiftedd maximum
1600 Jy
2.66 x 1044 s"1
15.77 km s"1
stellarr velocity
1700 Jy
2.66 x 1044 s" 1
Similarities between IRAS04553-6825 and NML Cyg
IRAS04553-68255 is remarkably similar to NML Cyg, a well known RSG in the Milky Way
(Johnsonn 1967; Diamond et al. 1984; Richards et al. 1996; Monnier et al. 1997), in terms
off luminosity, progenitor mass, spectral type, pulsation period, mass-loss rate, 10 //m feature
profile,, outflow velocity, CSE geometry, OH maser emission profile and peak flux density (scaled
too a common distance of 1 kpc), and SiO maser photon flux. The properties of the two stars
aree compared in Table 6.1. We compiled the infared photometry from the data provided in
Gezarii et al. (1993), and use F8„m(zeromag) = 2 x 10"12 W m" 2 ^™ - 1 = 42.7 Jy. The H 2 0
propertiess (NML Cyg: Yates et al. 1995) are also included in Table 6.1.
Thee pump efficiency of SiO masers is known to be related to the pulsation amplitude (Alcolea
ett al. 1990): Mira variables and semi-regular variables with visual amplitudes exceeding 2.5 mag
alwayss reach the maximum efficiency, while variables with smaller amplitudes are usually less
efficient.. We recalculated the pump efficiency of the SiO masers Fsio/F&tim. The pumping
inn IRAS04553-6825 is twice as efficient as in NML Cyg, but a factor ~ 4 less than in Mira
variables.. The SiO maser emission of IRAS04553-6825 has a larger ratio F i n t /F m a x than NML
Cygg (at a common resolution of 0.7 km s _1 ), and a few times as large as Mira variables (Alcolea
ett al. 1990; Chapter 5).
Thee near-infrared (JHKL) amplitudes are 0.4 mag for NML Cyg (Gezari et al. 1993) and
0.33 mag for IRAS04553-6825 (Wood et al. 1992). For NML Cyg, Kholopov et al. (1985) list
80 0
ChapterChapte
aa magnitude range of 11.19 to 12.54 mag in R, and 17.0 to 18.0 mag in V. We have sparse
opticall photometry of IRAS04553-6825 in the period 1994 to 1997, suggesting an amplitude
inn V of ~ 2 mag (smaller in R). We conclude that the pulsation properties of NML Cyg and
IRAS04553-68255 are similar, with possibly IRAS04553-6825 closer resembling Mira variables.
Fromm the similar properties of both objects we predicted to detect 22 GHz H 2 0 maser
emissionn from IRAS04553-6825 at a level of ~ 60 mJy (van Loon 1998). The measured peak
fluxflux density is in excellent agreement with this prediction.
6.3.44
Location of t h e H2O masers
AA striking difference between the H 2 0 masers of IRAS04553-6825 and NML Cyg is that in
IRAS04553-68255 the emission peaks at the stellar restframe velocity, whereas in NML Cyg the
linee profile resembles the OH maser line profile.
Interferometricc observations of the masers around NML Cyg indicate that its H 2 0 masers
aree amplified radially and that they are located in the dusty part of the CSE, where radiation
pressuree on the grains accelerates the outflowing matter (Richards et al. 1996). Indeed, in NML
Cygg the strongest emission occurs at a blueshift of ~ 20 km s _1 .
Thee peak of H 2 0 maser emission from IRAS04553-6825 falls within ~ 1 km s" 1 of the
stellarr velocity (as inferred from the SiO velocity), and all emission is detected within a range
off 8 km s _1 . This is similar to the situation in Mira variables, indicating low outflow velocities
inn the H 2 0 masing region of the CSE (Yates et al. 1995). In contrast, NML Cyg is similar to
thee OH/IR stars.
Cookee & Elitzur (1985) derive a scaling relation for estimating the inner radius rq of the H 2 0
masingg region, below which the density is higher than ~ 109 cm - 3 and the maser is quenched.
Thiss was confirmed by observations (Yates k Cohen 1994). The collision rate is dominated
byy collisions with H2 and does therefore not depend on metallicity: we therefore expect that
thee same relation will hold in the LMC. With an outflow velocity at rq of 0.8 km s _1 (half
thee FWHM of the H 2 0 maser peak), and an average particle mass of 2 x 10~24 g, we estimate
rrqq ~ 4 x 1015 cm, or ~ 20 times the stellar radius. This is outside the region where the SiO
maserr occurs (Humphreys et al. 1996).
Wee speculate that in IRAS04553-6825 the H 2 0 maser originates near the dust formation
radius,, where the logarithmic velocity gradient is very high (d(\n(v))/d(\n(r))Zl) and tangential
amplificationn dominates over radial amplification. Part of the emission may also originate from
insidee the dust formation radius where outflow velocities are low. Both cases could explain the
narroww H 2 0 peak coinciding with the tangentially amplified SiO maser peak (see also Engels
ett al. 1997). The H 2 0 masers in NML Cyg are located outside of the dust formation region,
wheree the outflow velocities are already £15 km s _1 (Richards et al. 1996).
Inn this model, either the maser occurs closer to the star in IRAS04553-6825, or dust
formationn takes place further out (as in galactic Miras as compared to galactic OH/IR stars).
Thee first possibility appears less likely if the inner radius is set by collisional quenching of
thee population inversion. It would be interesting to test whether at lower metallicity the dust
formationn and the acceleration of the outflow occurs at larger radii because of the relative lack
off refractory elements: in this case the difference between NML Cyg and IRAS04553-6825
couldd be a generic difference between similar stars in the Milky Way and the LMC.